A. Field of the Invention
The present invention generally relates to optical sensing means and particularly to temperature compensated sensing systems adapted for use in level-interface controllers and the like thereof.
B. Description of Prior Art
Several devices have heretofore been proposed and made commercially available for indicating level of material and the point of interface between materials in vessels or conduit. Some of the operation principles that these devices incorporate are: capacitance, floatation, paddle wheel, tuning fork, thermal sensing, and optical sensing.
U.S. Pat. No. 4,499,766 discloses a material level indicating device which uses a capacitive probe positioned within a vessel so that the electrical, and particularly the dielectric, characteristics sensed by the probe are indicative of the material being sensed. Capactive systems are typically used only in liquid level applications and are not usually available for extreme or fluctuating temperature service.
Another level measuring system incorporates a float that works on the principle of buoyancy. The liquid level fluctuations within the vessel cause the float to rise and fall between two or more user selected set points. When the float comes into contact with one of these set points a connection is made and an output is generated. Various means have been developed for interfacing the output indication and the float probe, which is typically inserted into a vessel. They typically include inductive, mechanical, or a similar linkage. This floatation design is plagued by linkage failure, particularly in sticky or viscous materials. It is also useful only in measuring liquid levels and can not be used in conduit or piping where the material is flowing or in interface detection.
Another device which makes use of a paddle wheel is inserted into a vessel in order to measure level and/or density. The paddle is normally connected to a motor which rotates it. This rotation is slowed or stopped when a material is present at the paddle, thus indicating the material's presence at that point. The paddle design is greatly hampered by mechanical failure, particularly due to the stress on the motor. In adddition, the design does not allow for sensing in flowing conduits and is not a reliable interface sensing system.
U.S. Pat. No. 3,625,058 discloses a device which utilizes a tuning fork to detect material level in a vessel. The fork is vibrated by a motor, an oscillating electromagnetic field, or some other similar active means. When different materials reach the fork the vibration changes are detected by sensing circuitry which recognizes a change in material density. The tuning fork design is useful only in vessels and is not accurate enough to give reliable interface measurements in most liquid applications, particularly when the viscosity of the various materials being detected is similar.
Thermal device designs have also been proposed to measure level and interface, particularly when the materials being sensed possess large variations in their thermal properties. As such, thermal designs make use of a probe which is inserted into a vessel at a desired point of measurement. Typically, the probe contains a heating element and temperature sensing element. When material reaches the probe tip, a temperature differential is produced due to the materials thermal conduction/dissipation properties. This differential causes the generation of a corresponding output signal external of the vessel which indicates the material level and/or identity. Although the design incorporates no moving parts, the heating elements themselves are prone to burn out failure. Furthermore, due to the time required for the slight temperature differentials involved to conduct through the probe, the response time for thermal sensing devices can be extremely long. This response time delay often makes the thermal design unusable, especially in real-time control loops and other time critical applications. Finally, the device can not be used for interface in a conduit where the material is flowing due to extremely rapid thermal dissipation. It is this principle that has led to the use of similar thermal designs in flow metering applications.
In the particular field of art where light is used, designs typically make use of differing indexes of refraction, such as U.S. Pat. No. 5,055,699, or incorporate reflective properties in design of the sensor assembly as in U.S. Pat. No. 5,164,606. These devices usually make use of glass, quartz, optical fiber, or similar light transmitters in which light generated from external of the vessel is piped/transmitted through the light transmitter. Depending upon the material's refractive index, the light either will or will not be reflected back to a light sensing element. Additionally, U.S. Pat. No. 4,694,161 shows a device in which optical fiber is used only as a convenient way of signaling a mechanically induced condition. Such a scheme does not, however, offer a technique for precisely detecting interface between materials, nor is it typically suitable for extreme industrial environments involving high pressure and temperature. The current light sensing assemblies do not account for variations in process and material temperature which degrades the accuracy of the sensing device.
It is well known that both photodiodes and photoresistors are affected by changes in temperature. Therefore, it would be desirable to have a sensing system which could detect material level and interface between materials that would be usable in extreme industrial environments and be accurate over a wide temperature range. Additionally, it would be desirable for such a system to be usable in conduits or piping in which the material being sensed is flowing, and for both solid and fluid service.